Multi-rotor UAVs with parallel thrust directions ill-suited for omni-directional interaction due to under-actuation, while a fully actuated configuration with non-parallel thrust directions will have a lower maximum upward thrust, reducing flight time. This paper aims to optimize thrust directions of a planar hexarotor for omni-directional interaction with minimal adaptations to the planar structure, to enable switching between and passively maintaining parallel thrust direction configuration and interaction configuration.

The thrust directions have been optimized for omni-directional interaction by maximizing the volumes of the force and torque spaces spanned by the thrust vectors. The directions of the thrust vectors are determined one parameter α which denotes the amount of rotation around the shaft connecting the rotor to the centre of the UAV. Two concepts for switch-and-lock mechanisms were designed and prototypes were tested. The first prototype uses a DC-motor to drive the mechanism and a worm wheel transmission with an end stop to lock the configurations. The other concept is driven by Shape Memory Alloy wire actuators (SMA wires) and locked using friction.

The results show that, when the drag torque has negligible effect on the volume of the torque space, the optimal angle isa* = 47.6°. When the drag torque is not negligible the optimal angle increases. This is especially relevant for UAVs with a radius smaller than 0.5 m. The prototype for the DC-motor concept delivered roughly 240mNm and was able to rotate the propeller assembly in under 0.55 s. The prototype for the SMA actuator did not produce more than 20 mNm due to limited pretension applied during manual wire installation. The maximum angle measured was 11°. Improving wire pretension can increase the generated torque and rotation angle, but it is not expected to reduce rotation time significantly, making the DC-motor driven concept more suited to drive a switch-and-lock mechanism for the omni-directional hexarotor UAV.

Planar hexarotor UAV optimized for omni-directional interaction can be switched to a parallel thrust direction configuration, for efficient flying, using a switch-and-lock mechanism consisting of a DC-motor, a worm wheel transmission and an end stop.